[update] update parallel examples in Java 8 module

brianway · brianway · commit c32220112835 · 2017-03-24T16:56:31.000+08:00
diff --git a/java8/README.md b/java8/README.md
@@ -39,6 +39,9 @@
 - CollectorHarness: 比较收集器的性能
 
 
+并行数据处理
 
-
-
+- ParallelStreamsHarness: 测量流性能
+- ParallelStreams: 并行流计算 1~n 的和,分别使用指令式,串行迭代流,并行迭代流,基本类型流,有副作用的流
+- ForkJoinSumCalculator: 使用分支/合并框架执行并行求和
+- WordCount: Spliterator: splitable iterator
diff --git a/java8/src/main/java/com/brianway/learning/java8/streamapi/parallel/ParallelStreams.java b/java8/src/main/java/com/brianway/learning/java8/streamapi/parallel/ParallelStreams.java
@@ -9,6 +9,9 @@
  */
 public class ParallelStreams {
 
+    /**
+     * for 循环的迭代
+     */
     public static long iterativeSum(long n) {
         long result = 0;
         for (long i = 0; i <= n; i++) {
@@ -21,10 +24,20 @@ public static long sequentialSum(long n) {
         return Stream.iterate(1L, i -> i + 1).limit(n).reduce(Long::sum).get();
     }
 
+    /**
+     * iterate 的问题:
+     * - iterate 生成的是装箱的对象,必须拆成数字才能求和
+     * - 很难把 iterate 分成多个独立块来并行执行
+     */
     public static long parallelSum(long n) {
         return Stream.iterate(1L, i -> i + 1).limit(n).parallel().reduce(Long::sum).get();
     }
 
+    /**
+     * 使用 LongStream.rangeClosed
+     * - 没有装箱拆箱
+     * - 容易拆分为独立的小块
+     */
     public static long rangedSum(long n) {
         return LongStream.rangeClosed(1, n).reduce(Long::sum).getAsLong();
     }
@@ -39,6 +52,9 @@ public static long sideEffectSum(long n) {
         return accumulator.total;
     }
 
+    /**
+     * 共享了可变状态,导致每次结果不一致
+     */
     public static long sideEffectParallelSum(long n) {
         Accumulator accumulator = new Accumulator();
         LongStream.rangeClosed(1, n).parallel().forEach(accumulator::add);
diff --git a/java8/src/main/java/com/brianway/learning/java8/streamapi/parallel/ParallelStreamsHarness.java b/java8/src/main/java/com/brianway/learning/java8/streamapi/parallel/ParallelStreamsHarness.java
@@ -3,6 +3,9 @@
 import java.util.concurrent.ForkJoinPool;
 import java.util.function.Function;
 
+/**
+ * 测量流性能
+ */
 public class ParallelStreamsHarness {
 
     public static final ForkJoinPool FORK_JOIN_POOL = new ForkJoinPool();
